"This study shows that your cardiovascular fitness at one point in time can predict how well your memory may function in the future," said Carrington Wendell.

Wendell led the study and is a researcher with the Bethesda, Maryland-based National Institute on Aging.

A growing body of research has hinted at a relationship between exercise and cognitive decline in old age. But prior studies typically measured physical fitness by asking people to recall how often they exercised in the past.

"Participants are not always the best historians," Wendell said.

For the new study, 1,400 men and women were asked to walk, jog or run on a treadmill until they were out of breath. A machine measured the amount of oxygen participants breathed in and carbon dioxide breathed out to calculate each person's so-called VO2 max.

"VO2 max is the maximal amount of oxygen used by your lungs during one minute of strenuous exercise. Generally, the more oxygen your lungs are able to use, the healthier you are," Wendell said.

She added that researchers working on similar studies in the past may not have chosen VO2 max as a measurement because it is time-intensive and can be expensive.

This relatively objective measure of fitness is "a particular strength of the study," said Stewart Longman in an email to Reuters Health. Longman is a rehabilitation psychologist at the University of Calgary in Canada and was not involved with the current research.

Participants were assessed when they were anywhere from 19 to 94 years old, as part of a study called the Baltimore Longitudinal Study of Aging.

The researchers followed each person for an average of seven years after the treadmill test. All participants took a memory test and followed up with the study team once, but less than half made a second visit to complete additional cognitive tests, the researchers wrote in The Journals of Gerontology: Series A.

"In terms of the level of evidence, this is an observational study," said Deborah Barnes. The study can show physical fitness is associated with better thinking and memory skills, but not prove it's responsible.

Barnes, a psychiatry researcher at the University of California, San Francisco, has studied exercise and cognitive ability but did not participate in the current study.

"Ten years ago, people were more skeptical about a relationship between exercise and the brain, but studies like this helps us realize that exercise has profound effects," Barnes said.

"The key message here is that being more physically fit may help someone keep their memory sharper with age," she said.

Wendell and her colleagues were not trying to determine why exercise might help prevent memory decline in their study. But they said past research suggests exercise may have a direct effect on signal-sending cells in the brain and other components of brain structure and function.

Researchers agreed future work on the subject still needs to be done.

"It would have been nice to have additional VO2 max measurements, instead of only at the beginning," Barnes said. With this extra data, researchers could have looked more closely at correlations between changes in aerobic fitness and cognitive performance over time.